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Rh which is not pleasant for the rider, especially as they try to pass on the outside, where a slip means tumbling down the precipice. Professor Hale had this path widened to five feet all the way before he could venture to send up the big five-foot mirror and its mounting; and I believe it is now being widened further still to twelve feet, so that they can carry up the loo-inch mirror in comfort when it is made. The five-foot has been at work on the top of the mountain for several years, and has taken some most beautiful photographs of objects far away in the depths of space. We will presently put some of them on the screen, and in that way we shall be practically allowing this magnificent telescope to carry us in imagination for a long "voyage in space" such as was not possible a few years ago. But it is a curious thing that when you get to the top of Mount Wilson, the Earth seems almost more striking than the heavens. Down below you in the plains are the two cities of Los Angeles and Pasadena; and when these are lit up at night they make glittering constellations of which the stars are brighter than those in the sky above. (See illustration facing p. 126.)

Having told you something about the history of these big reflecting telescopes—the biggest telescopes in the world—I want to go back and talk a little about the reason for making mirrors instead of lenses: so that the trouble about colour is avoided.

When we make a telescope with a lens we use a property of light called refraction, a word from the Latin which means simply breaking back. You know how a cricket ball can be bowled so as to go straight